Total hip arthroplasty (THA) is a predictably successful procedure for most low- or moderate-demand elderly patients, but its extension to even younger and more active patient groups has proven problematic, long-term. Recent survivorship data indicate that acetabular loosening, while tending to occur relatively later than that on the femoral side, poses a more ominous threat to construct longevity. Almost certainly, polyethylene wear debris is responsible for much of the problem. To date, clinical studies of THA wear have largely failed to rigorously address underlying mechanisms, whereas laboratory biomechanical studies largely lack long-term clinical validation. The principal investigator proposes an integrated series of complementary clinical and laboratory studies of THA wear, aimed at identifying causative factors and elucidating underlying mechanisms. Building upon experience from several previous studies, the investigator will draw heavily upon a unique information resource involving full serial clinical and radiographic follow-up records from 3,500 THA procedures performed by a single surgeon over a period of the last 20+ years. The applicant has developed a novel three-dimensional nonlinear finite element model which predicts (clinically realistic) rates and patterns of cup wear, based on contact stress and sliding distance. Concurrently, he has developed and physically validated a new procedure for very precisely measuring in vivo (radiographic) cup wear, using image analysis edge detection techniques. The complementary nature of these two new laboratory techniques and the long- term follow-up data will allow this group to test three major hypotheses: (1) The long-term in vivo wear performance of contemporary THA design variants can be predicted directly from their respective articulation kinetics; (2) Long- term polyethylene wear (and late loosening) can be predicted on the basis of early wear behavior; (3) In otherwise similar constructs, third-body particulate debris causes predictably accelerated polyethylene wear. The underlying objective of the proposed work is to develop reliable measures to predict long term polyethylene wear (and late acetabular loosening) on the basis of parameters observable in the short-term clinical setting and/or in laboratory models.